In the course of testing an internal combustion engine, it is known to inhibit the ignition of one or more cylinders in order to determine the contribution of such cylinder or cylinders to the total performance of the engine. An analysis of the resulting engine performance, e.g., variations in engine speed as obtained from a tachometer, reflects either individually, or as compared to other cylinders, mechanical operating conditions within the cylinders. However, these tests are normally conducted at a relatively low throttle setting such as at idle or fast idle conditions on an unloaded engine. An attempted extension of these techniques to nondestructive full throttle engine operation presents operational problems or hazards for both the vehicle and the mechanic.
Moreover, meaningful operational and diagnostic testing of the overall performance of internal combustion engines requires that tests include operation at or nearly at full fuel and air flow conditions, thereby exercising fuel and air intake systems at full capacity. Further, combustion chamber pressures must reach the relatively high, full-load or peak value in order for the presence of a malfunction to be more readily detected. If, however, the amount of fuel or air-fuel mixture delivered to an internal combustion engine, operating without a load, is increased beyond that required for maximum speed, the engine will overspeed. Further, if an engine is allowed to accelerate until it overspeeds, mechanical damage may result for the reason that the centrifugal and inertial forces on the moving parts exceed their designed values in this forbidden mode of operation. Moreover, an unloaded engine will overspeed long before the maximum fuel and air flow values are reached. Accordingly, to permit performance testing under these conditions a load must be connected to the engine under test.
In U.S. Pat. No. 3,757,570 to Richard T. Cowley and Leonard R. Hulls, and U.S. Pat. No. 3,757,571 to George A. Chamberas, both issued on Sept. 11, 1973, and assigned to the same assignee as the present invention, there are disclosed methods and apparatus for providing a simulated load for internal combustion engines. The simulated load is effected by periodically interrupting or inhibiting the engine ignition to reduce the total power of the engine to a value equal to the frictional horsepower at a desired speed. In pending application Ser. No. 346,671 filed on Mar. 30, 1973, now U.S. Pat. No. 3,839,907, issued on Oct. 8, 1974, by R. E. Hanson and H. E. Fineman an internal combustion engine is loaded in accordance with the teachings of the above-identified patents, making use of appropriate control interlocks to produce a simple operational test which protects the vehicle engine from possible inadvertent overspeed conditions, while associated apparatus analyzes the performance of the engine. The analyzer comprises means coupled to the ignition system for inhibiting the occurrence of ignition during a predetermined proportion of successively occurring ignition periods to provide a simulated load to said engine equivalent to a given actual load corresponding to given throttle setting of the resulting engine speed, between the minimum and maximum speed values, without applying an external load. The predetermined proportion of successively occurring ignition periods is selected to ostensibly provide a resulting nominal speed which is determined by the normal operating characteristics of the engine. Means are provided for detecting the actual speed of the engine which results from inhibiting the occurrence of the ignition during the predetermined proportion of the successively occurring ignition periods. Means coupled to the detecting means compare the detected actual speed of the engine to a reference speed related to the nominal speed to provide an output corresponding to the departure of the actual speed from the reference speed. Indicating means responsive to the output of the comparing means are provided for indicating the disparity between the acutal speed and the reference speed.